For many years in the past, paint primers for metal surfaces were prepared primarily from oil base paints. Within the past 10 or 15 years attempts have been made to prepare water base or latex paints for metal surface primers.
When these latex paints were used on rusty surfaces, rust spots would appear through the primer coat. Several anti-corrosion latex paints were developed to overcome the formation of rust spots which are called "flash-rusting", i.e., the rust spots would appear immediately on the paint surface.
Zinc oxide, zinc phosphate, zinc molybdate, zinc chromate, passive iron phosphate, iron oxide and basic lead-silica-chromate, among others, have been used as anti-corrosion agents with limited success.
None of these paint systems are able to prevent the formation of flash-rusting however, when iron or steel surfaces have been cleaned by blasting with iron grit or iron shot and the latex paint is used as a primer on such surfaces. None of the known anti-flash-rusting latex paints prevent rust spots from forming on the surface of the paint primer when used on such metal surfaces.
It is desirable, therefore, to prepare a latex paint primer which prevents flash-rusting when used on a metal surface which has been cleaned by iron grit or shot-blasting.